Up to 20% of all children have tics at some time in their life, and about 3% of all children have a chronic tic disorder such as Tourette syndrome (TS), making tic disorders a subject of substantial public health interest. Despite steadily increasing research, no treatment for TS works for more than half those treated, and the cause and pathophysiology of TS are poorly understood. Based on the observation that dopamine D2 receptor antagonists significantly reduce tic severity, one longstanding hypothesis has been that tics may involve abnormalities in transient (phasic) dopamine release in the striatum, while baseline (tonic) dopamine release may be normal. Several experiments in the past 15 years attempted to address this hypothesis by measuring striatal dopamine release in TS in response to amphetamine. One could argue, however, that this assessed only maximal possible dopamine release under nonphysiological conditions. The present proposal represents the first step in a plan to directly test phasic dopamine release in TS by measuring striatal dopamine release in response to a cognitive task with and without exogenous levodopa. The proposal will exploit the newly developed Siemens PET-MRI scanner to acquire rCBF simultaneously with the receptor imaging. The applicants have preliminary data on most aspects of this approach, considered individually, but none for the combined approach. This application proposes to test the full protocol on a small group of TS and matched control subjects, in order to demonstrate feasibility and estimate variance for a planned R01 application. The planned R01-funded follow-up study would include sample sizes adequate to test the effects of psychiatric comorbidity, past treatment, and demographic variables.